Quite some confusion on this one. CWGC has his name incorrect as Alexandra instead of Alexander (correct on SNWM). There is also a discrepancy in the name of his father between CWGC and the gravestone. However I am sure that it is the correct man, both full name and date of death agree.

I have reported the discrepancy of the names (Alexandra for Alexander) to CWGC today. His MM does not appear on the gravestone and he is not listed on the local war memorial, nor any civic memorial in Dumfries and Galloway.

This man is named on the Kirkmabreck Parish War Memorial in Creetown, Kirkcudbrightshire as:

MACDONALD, ALEXANDER SGT. CANAD'NS.
Alexander McMillan MacDonald, Meritorious Service Medal – age 29 – Sergeant (472155) 54th Battalion, Canadian Infantry.
Alexander had been a clerk at Creetown Station and had worked in the traffic manager's office in Stranraer before emigrating to Canada. He was a clerk, living at 108 31st Street West in Saskatoon, Saskatchewan, when he enlisted there in July 1915. He sailed from Halifax, Nova Scotia in June 1916 and went to France in August 1916, was promoted to Sergeant and was attached to the 4th Canadian Division Employment Company. He returned to his battalion in October 1917 but he was killed at Cambrai a year later. His award of the MSM was announced in January 1919.
(Source: Stewartry Roll of Honour, Service Record at the Library & Archives of Canada, the Saskatchewan Virtual War Memorial, London Gazette:17 January 1919 p.1018 and D&G Standard 19/10/1918 p.3)
Born 1889 in Dunscore, Dumfriesshire. Son of Robert Kinnear McDonald and Jessie (McMillan) McDonald of Cuil Woods Cottage and of Knockeans, Creetown. Husband of Vida MacDonald (latterly Johnson) of Toledo, Ohio, USA. [m.10/5/1916]
Killed in Action on 27 September 1918 and buried in Quarry Wood Cemetery, Sains-Les-Marquion, France.
Sains-les-Marquion is a village north-west of Cambrai. Near here on 27 September the battalion crossed the Canal du Nord and attacked and took Bourlon Wood and the village of Fontaine beyond. (Source:Battalion War Diary at the L&A of Canada)